Ducted fan gas turbine engines for powering aircraft conventionally comprise a core engine which drives a propulsive fan. The fan in turn, comprises a number of radially extending aerofoil blades mounted on a common hub and enclosed within a generally cylindrical casing.
There is a remote possibility with such engines that part or all of a fan blade could become detached from the remainder of the fan. This may occur as the result of, for example, the engine ingestion of a bird or other foreign body. It is, therefore, extremely important that the blade is retained within the casing and does not pass through and cause damage to the engine.
The use of containment rings for gas turbine engine casings is well known. Such rings have previously been manufactured from metal or alternatively glass fibre or carbon fibre, etc. They have normally formed an integral part of the compressor casing.
More recently the problem of fan containment has been addressed by winding strong fibrous material around a relatively thin fan casing. In the event that a fan casing becomes detached, it passes through the casing and is contained by the fibrous material.
The problem associated with such fibre wrap is that there is a danger that a blade could in certain circumstance cut though the fibre wrap and thereby pass straight through. This problem is addressed by GB 2159886B by the provision of fibrous patches positioned between the layers of material. The patches wrap around the blade during its passage through some of the material thus effectively blunting its leading edge and impeding it's progress through the remaining layers.
An additional difficulty with fan casing constructions is that in the interest of lightness the fan casing is made as thin as possible which leads to a lack of stiffness in the casing as a whole. This problem is particularly severe in large diameter fan casings. EP 0626 502 A1 discloses continuous lengths of material wound around rails which are mounted on the outer surface of the fan casing. The space between the rails is filled with discrete pieces of flexible material. Therefore, a detached blade initially breaks through the thin alloy casing, becomes `blunted` by the discrete pieces which become attached thereto and is then retained by the material wound around the rails.
However retaining the blade within the fibrous wrap can in some circumstances be difficult to achieve.